Method for enhancing v{11 ga thin film growth

ABSTRACT

A process for enhancing the growth of A15 intermetallic superconductors which removes impurities that are likely to form diffusion barriers impeding growth.

o i i United States Patent 1191 1111 3,811,185 Howe et a1. May 21, 1974[54] METHOD FOR ENHANCING V GA THIN 3,618,206 11/1971 Gubler et a1 .f.29/599 FILM GROWTH 3,625,662 12/1971 Roberts et a1. 3,713,898 l/1973Giorgi et a1 1 Inventors: David G Howe, Gre belt, d-; 3,728,165 4/19731166/1611 29/599 x Russell A. Meussner, Washington, 3,731,374 5/1973Suenaga et al..... D.C. 3,737,824 6/1973 Coies 174/D1G. 6 [73] Assignee:The United itatt: of America as FOREIGN PATENTS OR APPLICATIONS m f 'a gg: sgcetary 0 the 1,039,316 8/1966 Great Britain 29/599 [22] Filed: 1973Primary Examiner-Charles W. Lanham [21] Appl. No.: 344,402 AssistantExaminer-D. C. Reiley, 111

Attorney, Agent, or Firm-R. S. Sciascia; Arthur L. [52] US. Cl 29/599,29/199, 148/127, Branning l74/D1G. 6, 335/216 [51] Int. Cl HOlv 11/14[58] Field of Search 29/599, 199; 174/126 CP, S ACT 174mm 6; 335/216;148/127 A process for enhancing the growth of A15 intermetallicsuperconductors which removes impurities that [56] g gfig gif are likelyto form diffusion barriers impeding growth. 3,336,658 8/1967 Husni174/D1G. 6 7 Claims, 6 Drawing Figures FORM SHEATH ROD AND QB ENDPLUGHEAT 1 FORM ROD COMPOSITE SWAGE' SEAL UNDER VACUUM TENTEQMAY 21 I974 Kwfl PATENTEDIAY 2 1 I974 SHEET 2 OF 2 FORM COMPOSITE SEAL UNDER VACUUMETCH ROD SWAGE ANNEAL ROD HEAT FORM SHEATH ROD AND ENDPLUG FIG. 4.

hwzomezvwmwzvaik 89 s00 FORMATION TEMPERATURE PC) METHOD FOR ENHANCING V3 GA THIN GROWTH BACKGROUND OF THE INVENTION This invention relates tothe formation of superconductive materials more particularly it relatesto a novel process by which both the rate of growth of thesuperconductive material and the properties of said material magneticland transportation systems, naval propulsion systems, aircraft acgenerators, and large laboratory research magnets. Superconductinggenerators may be only one-tenth to one-third the size of conventionalelectrical generators of the same power rating. Savings are realized inoperating these systems when superconductive materials with highcritical temperature, critical current, and critical field are used.

The intermetallic A15 compounds, i.e. VGa, posses a high criticaltemperature and one of the highest critical fields as well as a goodcritical current-carrying capacity. However, the V Ga intermetalliccompound is extremely brittle andis very difficult to workmetallurgically. Recently, V Ga has been formed through a solid-statereaction process. The material is processed as a ductile Cu-Ga wire witha purevanadium core, and a V Ga interfacial layer is formed by ahightemperature reaction as the last step in the processing.Superconductors produced in this manner exhibit a critical temperatureof about 14K. By the process of the present invention superconductorsare formed which not only exhibit a critical temperature of about K, butexhibit a high rate of growth as well.

SUMMARY OF THE INVENTION Briefly the invention relates to a process ofeliminating any diffusion barrier between the interfacial layers of thesolid-state reactants.

It is therefore an object of the invention to provide a process by whichsuperconductive materials are produced.

It is another object of the invention to enhance the rate of growth ofthesuperconductive material.

Another object of the invention is to provide a method by which thesuperconductive material produced exhibits a high critical temperature,critical current and field.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a cross section ofan unassembled sheath, rod and end plug used to produce thesuperconductor.

FIG. 2 is an end view of the rod and the end plug.

FIG. 3 is a cross section of the sheath, rod and plug assembled as acomposite.

FIG. 4 is a flow chart describing the steps of the invention.

FIG. 5 illustrates a graphic comparison of the rate of growth of thesuperconductor produced in accordance with the present invention and therate of growth of a superconductor produced according to prior arttechniques.

FIG. 6 illustrates a graphic comparison of the critical temperature of asuperconductor produced in accordance with the present invention and thecritical temperature of a superconductor produced according to prior arttechniques.

DETAILED DESCRIPTION The invention relates to a novel process forproducing superconductive materials, such as V Ga, Nb Sn and V Si. FIG.1 illustrates in an unassembled state the elements used to produce thesuperconductor. The sheath, 10, provides a tapered bore 13 into whichthe core rod 11 and the copper end plug 12 are inserted as illustratedby FIG. 3. The diameter of the bore relative to that of the core rod issuch that when assembled an annular air space, 15, of about 0.005 inchesis created. The end plug 12 must be of sufficient length so that whenassembled as the composite illustrated by FIG. 3, the end plug extendsout of the sheath. The end plug is provided with axial grooves, 16,about its periphery so that air can be evacuated from the annular space15. The core rod 11 also has indentations or grooves, 14, at one end.These elements, the sheath, rod and end plug can be produced by an knowntechnique, such as are melting or induction melting.

While the end plug is copper, the composition of the sheath l0and corerod 11 are dependent upon the composition of the superconductor to beformed. The following table lists the compositions of the variouselements.

It can be seen then that the sheath is formed of a Cu alloy while thecore rod is formed from an alloy of the materials desired to form thesuperconductor. The composition of the core rod can be referred to as V-xGa, Nb-ySn or V-zSi. For best results, x ranges from 0 to 8 atomicpercent, y ranges from 0 to 2 atomic percent and z ranges from 0 to 4.5atomic percent.

Having formed the sheath, rod and end plug, the superconductor is thenformed by the process outlined in FIG. 4. First the rod is annealed at atemperature of about 800C for approximately 10 to 20 hours. The rod isthen cleaned by etching it with a nitric HF solution. A preferredsolution is 10 H O:l0 HNO 4HF. The compositeis then formed byplaciig'tlie'rod l l arid "end plug 12 within the sheath 10 as shown byFIG. 3, leaving an annular air space 15. The composite is placed underhigh vacuum in the range of 1 X 10 Torr or better so that the air space15 is evacuated and the composite is then sealed. The sealing can beaccomplished by welding the end plug to the sheath with the use of anelectron beam, laser and the like. By vacuum sealing the composite, anyair and other impurities which could cause a diffusion barrier areremoved.

The composite is then swaged or drawn so that it is reduced in diameter.This results in a co-axial wire which is either placed in an evacuatedsilica ampoule and heated in a furnace at constant temperature between550C and 880C or heated between 550 to 880C in an inert atmosphere underhigh vacuum. During this heat treatment the superconductor is formed orgrown" by diffusion occurring at the interface of the rod and sheath.

When the superconducting layer has been formed, the coaxial wire isremoved from the'furnace and is ready to be used as a superconductivematerial.

By providing the rod and end plug with grooves etching the rod to bothclean as well as form an active surface and sealing the composite undervacuum, the growth rate of the superconductor is enhanced.

FIG. 5 illustrates the results of various tests comparing the growthrate of superconductors formed according to the present invention andthat grown according to the technique taught by M. Suenaga and W. B.Sampson, Appl. Phys. Ltrs., 18:54, June 15, 1971. The plot relates thethickness of the superconductor, V Ga, versus time at a temperature of700C. Plot A refers to the prior art. Plots B and C are of thosesuperconductors formed by the present invention. The results of Plot Bwere taken from tests in which the core rod used was pure vanadium (V)while the results of plot C were taken from tests in which the core rodused was a V-Ga alloy in which 6.1 atomic percent Ga was present. Thegraph clearly shows that for any given period of time the rate of growthof the superconductor produced in accordance with the present inventionwas superior to that of the prior art. It also shows that the alloyedcore resulted in a higher rate of growth than the pure V COTE.

FIG. 6 illustrates the results of various tests comparing the criticaltemperature of V Ga grown by the process of the present invention andthat grown according to the Appl. Phys. Ltrs. article referred to above.Plot A refers to the prior art. Plot B refers to V Ga grown inaccordance with the invention in which a V-Ga core rod containing 6.1atomic percent Ga was used. The figure illustrates that criticaltemperature of V Ga grown by the process of the invention is higher thanthat of the prior art regardless of the formation temperature. v

The process of the invention results in not only a superconductor havingsuperior properties, but the rate of growth of the suprrconductor isalso superior to that of the prior art.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. lt is therefore to beunderstood that within the scope of the appended claimsthe-invention maybe practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

l. A method of forming a superconductor selected from the groupconsisting of V Ga, Nb sn, and V Si, respectively, said methodcomprising:

forming a sheath comprising an alloy selected from the group consistingof Cu-Ga, Cu-Sn, and Cu-Si, respectively;

forming a core rod having grooves at one end, said rod being composed ofan alloy selected from the 4 group consisting of V-xGa, Nb-ySn, andV-zSi, respectively, wherein x is 0 to 8 atomic percent, y is 0 to 2atomic percent and z is 0 to 4.5 atomic percent;

forming a copper end plug having axial grooves about its periphery;

annealing said core rod;

etching said rod with a nitric HF solution;

placing said rod and plug within said sheath whereby a composite isformed having an annular air space between said sheath and said rod,said plug being of sufiicient length to overlap the end of said sheath;

subjecting said composite to vacuum;

sealing said composite while under vacuum;

reducing the diameter of said composite; and

heating said reduced composite at 550C to 880C whereby saidsuperconductor is formed.

2. A method according to claim 1 wherein said annealing step is carriedout at 800C for about 10 to hours.

3. A method according to claim 1 wherein said nitric HF solution is 10 HO:10 HNO 4HF.

4. A method according to claim 1 wherein the composite is subjected to avacuum pressure of l X 10 Torr.

5. A method according to claim 1 wherein the composite is reduced indiameter by swaging.

6. A method according to claim 1 wherein the composite is reduced indiameter by drawing.

7. A method of forming a superconductor selected from the groupconsisting of V Ga, Nb Sn, and V Si,

respectively, said method comprising:

forming a sheath comprising an alloy selected from the group consistingof Cu-Ga, Cu-Sn, and Cu-Si, respectively;

forming a core rod having grooves at one end, said rod being composed ofan alloy selected from the group consisting of V-xGa, Nb-ySn, and V-zSi,respectively, wherein x is 0 to 8 atomic percent, y is 0 to 2 atomicpercent and z is 0 to 4.5 atomic percent;

forming a copper end plug having axial grooves about its periphery;

annealing said core rod at 800C for about 10 to 20 hours;

etching said rod with a solution of 10 H O:l0

I- INO 4HF;

placing said rod and plug within said sheath whereby a composite isformed having an annular air space between said sheath and said rod,said plug being of sufiicient length to overlap the end of said sheath;

subjecting said composite to vacuum at l X 10' Torr;

sealing said composite while under vacuum;

swaging said composite to reduce to diameter of said composite; and

heating said composite at 550C to 880C whereby said superconductor isformed.

2. A method according to claim 1 wherein said annealing step is carriedout at 800*C for about 10 to 20 hours.
 3. A method according to claim 1wherein said nitric HF solution is 10 H2O:10 HNO34HF.
 4. A methodaccording to claim 1 wherein the composite is subjected to a vacuumpressure of 1 X 10 5 Torr.
 5. A method according to claim 1 wherein thecomposite is reduced in diameter by swaginG.
 6. A method according toclaim 1 wherein the composite is reduced in diameter by drawing.
 7. Amethod of forming a superconductor selected from the group consisting ofV3Ga, Nb3Sn, and V3Si, respectively, said method comprising: forming asheath comprising an alloy selected from the group consisting of Cu-Ga,Cu-Sn, and Cu-Si, respectively; forming a core rod having grooves at oneend, said rod being composed of an alloy selected from the groupconsisting of V-xGa, Nb-ySn, and V-zSi, respectively, wherein x is 0 to8 atomic percent, y is 0 to 2 atomic percent and z is 0 to 4.5 atomicpercent; forming a copper end plug having axial grooves about itsperiphery; annealing said core rod at 800*C for about 10 to 20 hours;etching said rod with a solution of 10 H2O:10 HNO34HF; placing said rodand plug within said sheath whereby a composite is formed having anannular air space between said sheath and said rod, said plug being ofsufficient length to overlap the end of said sheath; subjecting saidcomposite to vacuum at 1 X 10 5 Torr; sealing said composite while undervacuum; swaging said composite to reduce to diameter of said composite;and heating said composite at 550*C to 880*C whereby said superconductoris formed.